Ive found that shadowboxing a combination for one round before and after hitting the bag really helps in nailing them down. For one round before hitting the heavybag Ill go as slow as possible and focus on technique and being relaxed. Ill speed it up so by the time I hit the heavy bag Im fully "warmed" up. For one round afterwards Ill shadowbox with an emphasis still on technique but Ill let my hands go as fast as I can without hurting my form. If a combination is new to me the last round Ill treat like the first because sometimes focusing on power and smashing the bag can make your technique suffer a little.